It is often desired to separate material by size, particularly in sand quarrying operations. It is known to provide a screening apparatus for such purpose, such apparatus typically comprising a frame or box, a motor assembly for vibrating the frame, a mesh screen mounted on the frame for separating materials by size and a feed device for delivering solid material, typically entrained or suspended in a flow of water, onto the screen, whereby fine solids, having a grain size smaller than the aperture size of the screen (known as undersize material), pass through the screen for collection/passage onto further processing steps, while solids having a grain size greater than the aperture size of the screen (known as oversize material) are conveyed over the screen to be collected from a downstream end of the screen. The feed device typically comprises a hopper or conveying device located above the screen whereby material to be screened, typically entrained or suspended in a flow of water, is dropped onto the upper surface of the screen.
The flow of water fluidizes the solid material, ensuring that the undersize and oversize materials are able to be separated efficiently while preventing the oversize material from clogging the screen. However, in known systems there is a tendency for the material to become dewatered while still on the screen due to the passage of water through the screen. Attempts have been made to alleviate this problem by spraying additional water onto the screen. However, such attempts result in increased water consumption and are generally not successful in re-fluidizing the solid material.